Moisture sorption measurements on plaster

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Moisture sorption measurements on plaster Jones, P. M.

NATIONAL RESEARCH COUNCIL OF CANADA D I V I S I O N OF BUILDING RESEARCH

MOISTURE SORPTION MEASUREMENTS ON PLASTER

bs

Peter

M a Jones

Report No.

167

of the D i v i s i o n of B u i l d i n g R e searah O t t a w a February

1959

PREFACE

Since water i s t h e most important a g e n t commonly involved i n problems of dimensions 1 change and d e t e r i o r a t i o n i n b u i l d i n g m a t e r i a l s , i n f o r m a t i o n on t h e e q u i l i b r i u m moisture c o n t e n t s of m a t e r i a l s i s of decided i n t e r e s t i n b u i l d i n g s c i e n c e . The D i v i s i o n has c o n s t r u c t e d , a s r e p o r t e d e a r l i e r i n DBR I n t e r n a l Report

133,

a s o r p t i o n b a l a n c e f o r d e t e r m i n a t i o n of s o r p t i o n isotherms. The a d d i t i o n a l u s e of a s m a l l o p t i c a l s t r a i n gauge which c a n be r e a d t h r o u g h t h e g l a s s w a l l s of t h e a p p a r a t u s h a s p e r m i t t e d simultaneous

measurement of l e n g t h ohange a t v a r i o u s e q u i l i b r i u m moisture c o n t e n t s . The anomalous behaviour of gypsum p l a s t e r i n some e a r l y experiments l e d t o t h e s t u d i e s whioh a r e now r e p o r t e d . These i n t u r n have drawn a t t e n t i o n t o t h e p o s s i b i l i t y t h a t

gypaum p l a s t e r may l o s e w a t e r of h y d r a t i o n under i n d o o r o l i m a t e o o n d i t i o n s of low r e l a t i v e humidity, and f u r t h e r s t u d i e s a r e now

i n

progress.

Ottawa

February 1959

N.B. Hutcheon

MOISTURE SOR PTION MEASUREMENTS ON PLASTER

P e t e r M. J o n e s

The n a t u r e of t h e a d s o r p t i o n of w a t e r on p l a s t e r a s p r e v i o u s l y determined and r e p o r t e d

i n

DBR I n t e r n a l Report No.

133

( l ) , was u n u s u a l and unexpected. The B.E.T. p l o t was n o t l i n e a r and, i n a d d i t i o n , t h e s u r f a c e a r e a c a l c u l a t e d from t h e t e s t r e s u l t s was u n u s u a l l y l a r g e . Both t h e i s o t h e r m and t h e c o r r e s p o n d i n g dimensional change i n d i c a t e d t h a t l a r g e changes o c c u r r e d i n t h e narrow r a n g e of v a l u e s of p/po betweon 0 and 0.1, and a g a i n when p/po of

0.8

was exoeeded. These r e s u l t s prompted s p e c u l a t i o n r e g a r d i n g p o s s i b l e mechanisms which would account f o r t h i s behaviour af p l a s t e r , a n d c o n s e q u e n t l y f u r t h e r work w a s undertaken.

The s t u d y d e s c r i b e d h e r e was d e s i g n e d t o . c o n f i r m t h e p r e v i o u s r e s u l t s and t o r e v e a l t h e p o s s i b l e mechanism. Samples were p r e p a r e d under normal c o n d i t i o n s of h y d r a t i o n , a s w e l l a a under c o n d i t i o n s which, i t was thought, would produce c o a r s e c r y s t a l s .of low s u r f a c e a r e a .

Apparatus and Sample 0

The a p p a r a t u s used was t h e same a s t h a t p r e v i o u s l y used,

t h e d e s c r i p t i o n and manner of o p e r a t i o n of which have been

r e p o r t e d ( I ) . Samples were prepared a s d e s c r i b e d i n t h e f o l l o w i n g paragraphs. I t should be n c t e d t h a t t h e o r i g i n a l c o n s t i t u e n t s and manner of mixing were t h e same i n a l l c a s e s , t h e d i f f e r e n c e s b e i n g i n t h e c o n d i t i o n s of s e t t i n g and subsequent t r e a t m e n t .

( a ) Unf i r e d ' "Hot" Sample

T h i s sample was p r e p a r e d by a d d i n g 530 grams of p o t t e r y p l a s t e r t o

318

grams of d i s t i l l e d water. F i v e seconds a f t e r a d d i t i o n , t h e m i x t u r e was s t i r r e d f o r

35

seconds, p l a c e d i n moulda, and allowed t o s e t a t

9S0c.

The r e s u l t i n g subatanoe was t h e n removed and s t o r e d i n a d e s i c a a t o r a t 50 p e r o e n t r e l a t i v e humidity f o r 1 month.

( b )

-

Unf i r e d

old"

Sample

T h i s sample was p r e p a r e d b y a d d i n g 530 grams of p o t t e r y p l a s t e r t o

318

grams o f d i s t i l l e d w a t e r . F i v e s e c o n d s a f t e r a d d i t i o n t h e m i x t u r e was s t i r r e d f o r

35

seoonds. T h l s m i x t u r e was p l a c e d i n moulds and a l l o w e d t o s e t a t room t e m p e r a t u r e .

It w a s t h e n removed and s t o r e d

i n

a d e s i c c a t o r a t 50 p e r c e n t r e l a t i v e h u m i d i t y f o r 1 month.

( c ) and ( d ) F i r e d Samples

P o r t i o n s of t h e " h o t " and " c o l d v samples were f i r e d a t

850

t o 9 0 0 ' ~ f o r 16 h o u r s , a l l o w e d t o c o o l , and s t o r e d

i n

a d e s i c c a t o r o v e r c a l c i u m c h l o r i d e u n t i l used.

R e s u l t s and D i s c u s s i o n

-

( a )

Unf i r e d Sa

E v a c u a t i o n of a d s o r b e d g a s e s f r o m t h e system r e q u i r e d

a

l o n g time. During t h i s e v a c u a t i o n t h e "hot" sample r a p i d l y l o s t 1 p e r c e n t of i t s w e i g h t , b u t t h e n remalnod u n a l t ' e r e d , w h i l e t h e " c o l d t t sample showed a c o n t i n u o u s l o s s o v e r a p e r i o d of t h r e e weeks u n t i l i t had l o s t a l m o s t 2 0 p e r c e n t of i t s weight. These r a t e s of l o s s a r e shown i n F i g . 1.

R e s u l t s o f t h e w a t e r vapour a d s o r p t i o n measurement a r e p l o t t e d i n Fig. 2 , B0E.T. c a l c u l a t i o n s were made on t h e s e

samples and p l o t s a r e shown i n F i s.

3

and

4.

The s u r a c e a r e a s

8

?i

c a l c u l a t e d from t h e s e p l o t s a r e 1

6.1

rn2/g and

16.73

m /g f o r the

"cold'' and "hotl1 s a m p l e s r e s p e c t i v e l y .

I n t h e c a s e of t h e " c o l d " sample, t h e a d s o r p t i o n i s o t h e r m shows two p o r t i o n s where a l a r g e i n c r e a s e i n w e i g h t s o c c u r s ,

between 0 and 0.1 v a l u e s of p/po and when p/po of

0.8

i s exceeded. The B0E.T. p l o t does n o t g i v e a s t r a i g h t l i n e , b u t n e v e r t h e l e s s on c a l c u l a t i n g t h e s u r f a c e a r e a of t h e sample a v e r y l a r g e v a l u e i s f o u n d ,

166.1

m2/g. The s u r f a c e a r e a of c o n c r e t e h a s p r e v i o u s l y b e e n f o u n d t o be

43

m2/g.

P l a s t e r i s t h o u g h t t o b e i n a c r y s t a l l i n e f o r m when s o l i d , whereas c o n c r e t e i s t h o u g h t to be a g e l , and i t

would h a r d l y be e x p e c t e d t h a t a c r y s t a l l i n e s t r u c t u r e would have a l a r g e r s u r f a c e a r e a t h a n a g e l .

It i s t h u s p o s t u l a t e d t h a t t h e measured i n c r e a s e - i n w e i g h t i s n o t all p h y s i c a l a d s o r p t i o n , b u t p a r t c h e m i s o r p t i o n r e s u l t i n g i n h y d r a t i o n . T h i s t h o u g h t i s s u p p o r t e d by a c l o s e r

s t u d y of the r e s u l t s . Twenty p e r c e n t by w e i g h t o f w a t e r was removed on d e g a s s i n g which c o r r e s p o n d s t o t h e t h e o r e t i c a l w a t e r

of h y d r a t i o n of 2 1 p e r c e n t . T h i s d i f f e r e n c e i s p r o b a b l y due t o t h e f a c t t h a t t h e s a m p l e was n o t p u r e CaSO4. 2H20.

The l a r g e i n c r e a s e between 0 and 0.1 v a l u e s of p/po i s

6

por c e n t and appoarv t o c o r r e s p o n d w i t h a

6.6

p e r c e n t i n c r e a s e i n weight when anhydrous CaS04 i s h y d r a t e d t o ~aS04.1/2 H20.

The second l a r g e i n c r e a s e of 23 p e r c e n t when p/po v a l u e of 0.8 i s exceedod, a p p e a r s comparable w i t h complete h y d r a t i o n t o CaSO .2H 0 which p r o d u c e s 26 p e r c e n t i n c r e a s e i n weight.

₄

2

Thus i t i s p o s t u l a t e d t h a t t h e i n i t i a l i n c r e a s e of

6

p e r c e n t d u r i n g a d s o r p t i o n c o r r e s p o n d s t o h y d r a t i o n from t h e anhydrous CaSO t o t h e hemihydrate CaS04.1/2 H20. P h y s i c a l

4

a d s o r p t i o n a p p e a r s t o t a k e p l a c e on t h i s m a t e r i a l u n t i l t h e

second l a r g e i n c r e a s e o c c u r s . I t i s thought t h a t t h i s c o r r e s p o n d s t o h y d r a t i o n from t h e hemihydrate t o t h e d i h y d r a t e .

C a l c u l a t i o n s and B.E.T. p l o t s u s i n g t h e l i n e a r form of t h e e q u a t i o n were m d e on t h e p o s t u l a t e d a d s o r p t i o n p o r t i o n of t h e c u r v e , and a r e g i v e n i n F i The approximate v a l u e of t h e

s u r f a c e a r e a was found t o be 2%:b5i2/go which i s a more r e a l i s t i c f i g u r e when compared t o t h e v a l u e f o r c o n c r e t e .

The d i m e n s i o n a l change was v e r y l a r g e and was beyond t h e r a n g e of t h e e x t e n s o m e t e r . But i t a p p e a r s t h a t v a l u e s o b t a i n e d show t h e t e n d e n c i e s e x h i b i t e d i n t h e weight change ( F i g .

6 ) .

I n t h e c a s e of t h e " h o t w sample, s e p a r a t e e x p e r i m e n t a t i o n by f i r i n g shovred t h a t the o r i g i n a l sample c o n t a i n e d 2.54 p e r c e n t t o t a l w a t e r c o n t e n t and s o was n o t i n t h e form of t h e hemihydrate. During the d e g a s s i n g p e r i o d t h e sample l o s t 1 p e r c e n t by weight and r e - a d s o r b e d t h i s on a d s o r p t i o n , b u t i t showed no tendency t o - h y d r a t e a s d i d t h e "cold1' sample.

No d i m e n s i o n a l change was made on t h e " h o t " sample. F i r e d Samples

It was hoped t h a t t h e s e samples would have d i f f e r e n t s t r u c t u r e s and t h a t they would be s u i t a b l e f o r p u r e a d s o r p t i o n measurements w i t h o u t any h y d r a t i o n . The i s o t h e r m s a r e g i v e n

i n F i g ,

7

and t h e d i m e n s i o n a l change r e s u l t s i n Fig. 8. I t i s s e e n t h a t t h e a d s o r p t i o n i s v e r y s m a l l indeed. No B o E o T o

c a l c u l a t i o n s were made and s o no s u r f a c e a r e a measurements were p o s s i b l e , The d i m e n s i o n a l changes a r e a l s o of a v e r y s m a l l o r d e r .

C o n c l u s i o n s

-

I t i s a p p a r e n t t h a t t h e a d s o r p t i o n c h a r a c t e r i s t i o s of p l a s t e r cannot be s t u d i e d i n t h i s a p p a r a t u s , beoause of t h e decomposition t h a t o c c u r s a t h i g h vacuum. I t i s p o s t u l a t e d t h a t d u r i n g vacuum d r y i n g and subsequent exposure t o i n c r e a s i n g huniidit i e s i n 1;he t e s t a p p a r a t u s

i t

d e h y d r a t e s and t h e n rehydra t e s

i n t h r e e s t a g e s :

( 1) h y d r a t i o n t o hemihydrate CaS04.1/2 H20

( 2 ) p h y s i o a l a d s o r p t i o n on t h e hemihydra t e

( 3 )

hydra t i o n t o t h e d i h y d r a t e CaS04.2H20.

The " h o t " sample shows no t e n d e n c y t o h y d r a t e , which r a i s e e t h e f o l l o w i n g q u e s t i o n s :

(1) what i s t h e c o n a t i t u t i o n of t h i s ''hotW p l a s t e r ? ( 2 ) why does n o hydra t i o n t a k e p l a c e ?

The f i r i n g o f t h e samples d i d n o t produce samples on which t h e pure a d s o r p t i o n i s o t h e r m s c o u l d be measured b u t produced samples which had undergone p h y s i c a l changes r e s u l t i n g from e i t h e r f u s i n g of t h e samples, which gave low s u r f a c e a r e a o r some o t h e r a o t i o n which r e n d e r e d t h e samples i n e r t t o w a t e r a d s o r p t i o n .

Ref a r e n c e

1. Mossman, C.E. Moisture a d a o r p t i o n measurements. N a t i o n a l R e s e a r c h Counc 11, D i v i s i o n of B u i l d i n g Re s e a r c h , DBR I n t e r n a l R e p o r t

133.

Ottawa, October

1957.

0 COLD SAMPLE HOT SAMPLE

FIGURE I

0

8 lo 12 14 16 18 2 0 -

REMOVAL

OF

H,O

FROM PLASTER SAMPLE BY

EVACUATION AT

mm.

Hg

-

T\'

~ - ~ - Q - O - @ - ~ - O - O - O - O - ( ~ - ~ - ~ ~ ~

I

I

I

I

I

I

I

2-

0 4 -

\

-

-

0 6 -

-

-

\*

-

-

\

-

-

"\"

-

-

\

"\

-

-

-

-

O \

"\

O \

-

O-0,"

I

1

I

I

I

I

I

I

0 2 4 6 8 10 12 14 16 18 DAYS OF PUMPING AT rnm. HCJ.

0 COLD SAMPLE

HOT SAMPLE

FIGURE 2

FIGURE

4

B.E.T.

PLOT ON

"COLD" PLASTER (ASSUMING NO

HYDRATION)

USING ADSORPTION

CURVE. CALCULATED

FIGURE 5

B.E.T.

PLOT

ON

"COLD" SAMPLE

(

ASSUMING

HYDRATION

OCCURS).

CALCULATED

SURFACE

AREA

=

28.42 m

/

_-9-

0 FIRED COLD SAMPLE

@ FIRED HOT SAMPLE

FIGURE

7

ADSORPTION

O F WATER

VAPOUR ON FIRED P L A S T E R

S A M P L E S

DIMENSIONAL

CHANGE

OF FIRED COLD PLASTER

SAMPLE